Wastewater treatment by contact with activated sludge is well-known. In a typical wastewater treatment plant, the influent passes through a headworks for screening and grit removal and then a series of treatment processes. Screening removes roots, rags, cans and large debris. Grit is removed in a quiescent section of the headworks. Pre-aeration freshens wastewater and helps remove oil. This primary treatment of sedimentation and flotation removes settleable and floatable materials. A secondary treatment of blending the raw influent wastewater with return activated sludge biologically stabilizes wastewater by removing the suspended and dissolved solids. This activated sludge contains microorganisms which assimilate the waste materials. Disinfection kills pathogenic organisms in the clarified wastewater. The resulting effluent is then generally discharged to surface waters.
Upon entering the secondary treatment process, the raw wastewater is mixed in a first aeration tank with return activated sludge which typically comprises relatively high concentrations of microorganisms. The return activated sludge comes from a secondary clarifier, as discussed below. The mixture is aerated and agitated in a series of aeration tanks to facilitate the growth of the sludge. The addition of air induces the growth of the microorganisms living in the sludge. The microorganism, such as bacteria, fungi, and protozoa, feed on the raw wastewater to reduce and decompose the wastes in the wastewater. The aeration process is approximately 24 hours.
Following aeration, the mixture is allowed to settle in the secondary clarifier. The microorganisms and waste collect into larger clumps of material known as floc. The activated sludge floc separates from the water by gravitational force due to its higher specific gravity and settles on the bottom of the secondary clarifier. The water on the surface is removed to a disinfecting tank. This disinfected water is the plant effluent and is ready for disposal by dilution or direct discharge to surface waters. The activated sludge from the bottom of the secondary clarifier, known as return activated sludge, is pumped to the first aeration tank for mixture with the raw influent wastewater, thus completing the cycle.
Activated sludge is typically measured in terms of biochemical oxygen demand (BOD) in milligrams per liter (mg/l), which is the strength of the wastewater and primary food source for the microorganisms, and total suspended solids (TSS). Domestic raw wastewater typically is 250 mg/l BOD and 200 mg/l TSS. The return activated sludge solids concentration is typically between 2000-6000 mg/l TSS. The plant effluent is typically 10 mg/l for both TSS and BOD. The activated sludge continues to grow by assimilating waste products as it passes through the process.
Sludge dewatering may be accomplished by several methods, such as drying beds, sludge lagoons, withdrawal of wet sludge to land as topical fertilizer, and mechanical apparatus such as vacuum filters and centrifuges. Small capacity wastewater plants typically use drying beds for dewatering sludge. The drying process occurs by evaporation and percolation of the water from the sludge. Typical drying beds are 15 to 18 inches in height and have a drainage system under the bottom of the bed. The drainage system typically has a layer of coarse crushed rock, a layer of gravel, a layer of pea gravel and a cover layer of 6 to 8inches of sand. The sludge is applied on top of the sand to a depth of approximately 12 to 14 inches. The drying time in warm weather is typically 4 weeks. Rain or other precipitation increases the drying period. Dried sludge is removed from the bed manually or with heavy equipment and may be used as a low grade fertilizer. The drainage water from the drying beds is typically returned to the headworks. The quantity of dried sludge produced is typically 60% of each unit of influent wastewater BOD treated.
Wastewater treatment plants utilizing drying beds have large spacial requirements in that as waste sludge is produced need to access drying beds is required. These plants are extremely susceptible to meteorological conditions. If it rains, the sludge rehydrates and requires increased drying time to remove the additional water. With large amounts of rain, the sludge will not dry. After drying the sludge is removed manually for disposal. Sludge is typically removed from the beds by a shovel.
Thus, a need exists for a wastewater treatment process that does not generate excessive sludge and has an effective sludge dewatering system which is not impacted by climate conditions.